Re: C library promoted to C++ linkage due to optional C++ source module

[Bob Friesenhahn]

> Hi Bob,
>
>      It is my opinion that if a library or program
>      is linked with C++ libraries (and especially if it is a static build!)
>      that it should be linked using the C++ linker.  Likewise, if a library
>      or program does not depend on, or contain any C++ code, it should be
>      linked with the C linker.
>
> I surely agree, in principle. But that doesn't mean I have the faintest
> idea what to change in Automake to make that be the case. You probably
> know better than me :). Can you provide (as simple as possible) test cases?

I have since backed away from this approach because I saw that the 
clever way that Automake handles conditionals is not going to be smart 
enough to make language decisions based on if a file is added or 
removed.  The compiler/linker type to use is already baked into 
Makefile.in and the choice used is also based on the optional source 
files.  If a C++ file appears, the language is promoted from C to C++ 
(--tag=CXX) and is baked into Makefile.in.

I see code like this appearing in Makefile.in:

Magick___lib_libGraphicsMagick___la_LINK = $(LIBTOOL) $(AM_V_lt) \
    --tag=CXX $(AM_LIBTOOLFLAGS) $(LIBTOOLFLAGS) --mode=link \
    $(CXXLD) $(AM_CXXFLAGS) $(CXXFLAGS) \
    $(Magick___lib_libGraphicsMagick___la_LDFLAGS) $(LDFLAGS) -o \
    $@

As such, it seems that this would be a major feature request and 
significant redesign, and is not a simple bug.

I may be possible to replace this Makefile hunk as an override, but it 
may not be reliable.

It does seem important to link with the C++ compiler when depending on 
C++ libraries, but modern Linux apparently works "fine" if you don't.

>      I have a continuing problem that when libtool tests
>
> bug-libtool@gnu.org for that one, I think ... --best, karl.
There is a bug open already in libtool for the issue with dropping -lm 
when mixing C and C++.  It is not clear that the C++ linker provides -lm 
by default since it seemed to be pulled in by implicit dependencies as 
part of the libraries implementation.

Bob

--
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer,    http://www.GraphicsMagick.org/
Public Key,     http://www.simplesystems.org/users/bfriesen/public-key.txt